1. Field of the Invention
The present invention relates generally to a circuit substrate and particularly to a composite substrate on which, for instance, semiconductor chips are mounted, and its production. More specifically, this invention is concerned with a high-density mounting multi-layer substrate on which materials for conductors, resistors, capacitors, etc., are concurrently fired or otherwise thermally treated, and its production.
2. Prior Art
So far, alumina substrates have been used as circuit substrates. Now that a delay in the transmission speed of signals through wires that make connections between semiconductor elements has been not negligible with an increase in high-speed proceedings by such elements, however, the alumina substrates are said to be unfit for use with high-frequency circuits, etc., because of their high dielectric constants that, for instance, are 9.5 at 1 MHz. In other words, the dielectric constant of substrate material is desired to be low for enabling signals to be transmitted at high speeds. There, too, are some other demands for substrate material that it is so close in terms of the coefficient of thermal expansion to silicon that naked elements can be mounted directly on the substrate at high densities, and that it can be sintered at a low temperature of 1000.degree. C. or below so that it can be fired or otherwise thermally heated along with electrode material having a low melting point such as Cu or Au.
To meet such demands, intensitive and extensive investigations are now in the making. A typical proposal is made of a glass sintered member obtained by firing or otherwise thermally treating a compact made up of powders of a glass composition containing SiO.sub.2, Al.sub.2 O.sub.3 and MgO as main ingredients, characterized in that the glass composition comprises 45 to 60% by weight of SiO.sub.2, 10 to 25% by weight of Al.sub.2 O.sub.3, 25 to 40% by weight of MgO and up to 5% by weight of at least one nucleator selected from the group consisting of TiO.sub.2, ZrO.sub.2, SnO.sub.2, P.sub.2 O.sub.3, ZnO, As.sub.2 O.sub.3 and MoO.sub.3 (see JP-U-62-252340).
JP-U-62-252340 states that the sintered glass member can be fired concurrently with a low-resistant conductor metal such as silver, silver-palladium or gold, is unlikely to degrade in terms of insulating properties due to a migration phenomenon even when used in the form of multi-layer circuit board substrate material, and has a low dielectric constant.
However, a serious problem with a substrate formed of this sintered glass member is that its strength properties are less than satisfactory.
It is therefore an object of the invention to provide a substrate that has good-enough mechanical strength, when formed into a circuit substrate, has a coefficient of thermal expansion close to that of silicon and a low dielectric constant, and can be fired or otherwise thermally treated at low temperatures.